Group III-V semiconductor devices are of wide interest and application. These devices have many advantages over silicon based semiconductors, including speed and an ability to produces optical emissions. They are widely used in optoelectronic devices. One problem common to Group III-V devices relates to the electrical contact metallizations used to form interconnects between discrete devices. Resistivity is obviously an issue, since lower contact resistivities lower power consumption and decrease heat generation. Bonding is also an issue, since failure to form a reliable bond with the III-V material will result in devices prone to poor operation and short life span.
A particular example is the prior use of AuGeNiAu as an emitter contacting layer on the GaAs emitter caps of GaAs heterojunction bipolar transistors (HBTs). Experience has shown these contacts to be unreliable due to Au spiking and balling. Au rapidly diffuses into GaAs and nonplanar surface morphologies often result. These problems became more prominent as feature size was reduced. These problems led to modem conventional HBTs, which typically employ a non-alloyed TiPtAu emitter contact on a strained InGaAs contacting layer to eliminate the possibility of spiking. Fabrication is rendered difficult, however. Strained InGaAs causes difficulty during crystal growth and during a self-aligned wet chemical etch.
Others have investigated the use of PdGe contacts to alleviate these and similar problems. For example, Lai et al. have reported PdGe contacts on n-type GaAs and InP by rapid thermal annealing. See, "Pd/Ge ohmic contacts to n-type GaAs formed by rapid thermal annealing", Appl. Phys. Lett. 64 (2), p. 229, Jan. 10, 1994; and "Low Contact-Resistance and Shallow Pd/Ge Ohmic Contacts to n-In.sub.0.53 Ga.sub.0.47 As on InP Substrate Formed by Rapid Thermal Annealing", Jpn. J. App. Phys. Vol. 35 (1996) Pt. 2, No. 12A, p. 1569. Additional work has been done by Marshall et al., "Nonalloyed ohmic contacts to n-GaAs by solid-phase epitaxy of Ge", J. Appl. Phys. 62 (3), p. 942 (Aug. 1, 1987). Good resistivities were obtained by these and other previous works. Contacts according to many of the published works are revealed to be highly reactive, though, indicating poor suitability for manufactured devices. In addition, though the most commonly reported resistivities of 10.sup.-5 -10.sup.-6 .OMEGA.-cm.sup.2 are ohmic, even lower resistivities are always better.
Accordingly, there is a need for an improved method for forming PdGe contacts on Group III-V layers. There is a further need for an improved method for forming PdGe contacts on Group III-V layers that lowers the etch reactivity of the contacts, provides good resistivity, and avoids manufacturing difficulties such as strained intermediate layers. There is an additional need for an improved PdGe-Group III-V direct interface.